This invention relates to diode phase shifters implemented with striplines. More specifically, the invention relates to a stripline diode phase shifter which has a low line loss and is miniaturized.
Conventional hybrid couplers employ first and second juxtaposed hybrid coupling line conductors with an input signal applied to one end of one conductor and an output signal taken from the far end of the other conductor. Thus, for example, the left-most hybrid coupler 6 in FIG. 1 includes a lower conductor which receives an input signal at its left end and an upper conductor which provides an output at its right end. If the coupling between upper and lower conductors is weak, the input signal will be transmitted down to the right end of the lower conductor, which is referred to as a transmitting terminal. If the coupling between upper and lower conductors is strong, the signal will be split into two parts with one part appearing at the transmitting terminal and the other part being coupled over to the left end of the upper conductor, which is therefore referred to as the coupling terminal. The signals at the transmitting and coupling terminals will be 90 out-of-phase, and the coupled signal will travel down to the output terminal. The amount of phase shift provided by the coupler is typically controlled by connecting phase-shifting diodes to the coupling and transmitting terminals and varying the bias applied to the diodes. Thus, these coupling and transmitting terminals may also be referred to as phase-shifting terminals.
In general, plural diode phase shifters are cascade-connected so as to provide a multi-bit phase shifter. FIG. 1 is a schematic circuit diagram of a conventional 3-bit stripline diode phase shifter. In FIG. 1, an electromagnetic wave applied to a main line 5 from an RF input terminal passes through a DC cut or block 7 which blocks a DC bias component and permits the passage of only RF components thereby isolating the DC bias from one another and from the external circuit. The DC cut 7 is a wide bandpass filter which is constructed such that the coupling terminal and the transmitting terminal of a coupling line-type hybrid having a length corresponding to a 1/4 wavelength are open and that at a frequency near its designed center frequency, the characteristic impedance of the D.C. cut is equal to the characteristic impedance of a hybrid 6. Thus, the DC cut 7 is formed as a quarter-wave hybrid coupler whose length is equal to a 1/4 wavelength. Thereafter, the electromagnetic wave passing through the DC cut 7 is applied to a hybrid 6 which acts as a 180.degree. phase shifter and is then transmitted to the main line 5 on the output side through further DC cuts 7, a 90.degree. phase shifter and a 45.degree. phase shifter in succession. Each of the phase shifters is operated by switching the polarity of a DC bias applied to its corresponding bias circuit 9 and the DC biases are isolated from one another by DC cuts 7 as described above.
As is well known in the art, phase shifting circuits of this type are formed by printing stripline conductors on opposite surfaces of a substrate and then placing dielectric layers over each of the printed circuits. Finally, ground layers are disposed over each of the outer dielectric layers. The resulting structure consists, in order, of ground, dielectric, printed circuit, dielectric, printed circuit, dielectric, and ground layers, with the center dielectric being referred to as the "intermediate layer substrate" in the present application.
FIG. 6(A) shows a diode loading line which is provided on the intermediate layer substrate of the phase shifter. As is shown in FIG. 6(A), the diode loading line includes a .lambda./4 impedance conversion line section 20 and a diode loading line section 21, the respective impedances Z.sub.1 and Z.sub.2 of which are so designed that a desired amount of phase shift is provided and that the difference in loss is minimized when the bias is switched. Furthermore, in FIG. 6(A), the length .theta. of the diode loading line section 21 is so selected that at the connecting point of the diode loading line section 21 and the .lambda./4 impedance conversion line section 20 the phase of reflection voltage from the diode is equal in magnitude but different in sign when the bias applied to the diode is switched.
The above-described conventional stripline diode phase shifter is advantageous in that a wide band characteristic can be obtained. However, it is disadvantageous in that, since the hybrids 6 and the DC cuts 7 are alternately cascade-connected as illustrated in FIG. 1, reflections are quite high in the main line, the loss is correspondingly increased, and the overall longitudinal dimension of the device is necessarily long.